…xtract-plugin

Port additional regression / edge-case fixtures that exist in
webpack-contrib/css-loader and webpack-contrib/mini-css-extract-plugin
but were not yet covered by webpack core's built-in CSS support.

css-loader:
- file.with.many.dots.in.name.module.css (issue-980): file name with
  multiple dots produces a valid local ident
- composes-duplicate.module.css: same imported class composed in
  multiple selectors of the same module
- keyframes-leak-scope.module.css: @Keyframes with default :local,
  :global(...), :local(...) and animation/animation-name interaction
- path-placeholder.module.css (issue-967): selectors with escaped
  filesystem-reserved chars

mini-css-extract-plugin:
- at-import-in-the-entry: external @import in an entry CSS that is
  imported after another entry CSS
- at-import-chain: deep chain of local @imports
- content-entries-with-same-import: two async chunks importing the
  same CSS files in different orders

Mini-css-extract-plugin's insert option is replaced in webpack core by
the createStylesheet compilation hook on CssLoadingRuntimeModule, which
lets a plugin patch the link element creation code. Add a test that
demonstrates this and verifies the customization reaches the runtime.

Also port two HMR cases from mini-css-extract-plugin that webpack core
did not yet exercise:

- imported-css-chain: HMR through @import a.css <- c.css <- index.css
  with a sibling b.css, mirrors mini-css-extract-plugin's hmr fixture
- composes: HMR for a CSS module that composes from a sibling module,
  asserting both the link sheet content and the composed locals stay
  consistent across the update

… ignoreWarnings

css-loader directory:
- localIdentHashSalt alone, localIdentHashDigest = "hex"/"base64",
  localIdentHashFunction = "md4" + "base64url", and a combined
  salt + sha256 + hex + custom length variant. These exercise the
  rename of css-loader's hashSalt option to localIdentHashSalt and
  cover digest variants the existing fixture didn't reach.
- at-value-extra.module.css: missing @value scenarios that aren't in
  css-modules/at-rule-value.module.css — @value composing imported
  values, @value inside func() positions, @value imported from a
  node_modules package (raw and via tilde alias), and a multi-step
  identifier chain.

ignore-conflicting-order:
- Same CSS chunk-merge that conflicting-order produces a warning
  for, but with ignoreWarnings: [/Conflicting order/] in the config.
  Verifies the warning is suppressed (no warnings.js fixture to
  match against).

…ests

Lets the generator.exportsConvention function form return an array of
names so a single class is exported under every alias in the array,
mirroring css-loader's exportLocalsConvention function behaviour.
Single-string returns continue to work unchanged.

Tests:
- exports-convention-function-array: function returning [name, name.toUpperCase()]
  exports both lower- and upper-case keys for the same class
- createStylesheet-attributes: covers mini-css-extract-plugin's `attributes`
  and `linkType` plugin options by tapping CssLoadingRuntimeModule's
  createStylesheet hook to set id/data-* attributes and link.type
- sync-class-names-between-targets: uses [file]__[local] localIdentName
  so web and node compilations produce identical class names with no
  getJSON-style cross-target sync needed
- hotCases/css/hmr-extra-links: ports the unrelated-link scenarios from
  mini-css-extract-plugin's HMR.test.js (data:, # anchor, link without
  href, external stylesheet) so CSS HMR is shown to leave them alone
  across multiple sequential updates